Physical adsorption in disordered porous materials: Theory and computer simulations
This thesis is aimed to gain a better understanding of adsorption phenomena, with a special emphasis on hysteresis, an effect commonly observed in adsorption experiments on porous substrates. We approach this problem using an array of statistical mechanical methods and computer simulation techniques. Using a generic model of a disordered porous material we were able to show that disorder plays an important role in confined fluid phase behavior. On the other hand there is an intrinsic link between the shape of the phase diagrams for confined fluids and corresponding adsorption isotherm. For a model of a disordered porous material we have shown that adsorption isotherms are in a good qualitative agreement with experimental data on adsorption of nitrogen and inert gases in silica xerogels, specifically in the hysteresis region. This agreement was further explored, using a molecular dynamics technique that closely mimics diffusive mass transfer mechanisms of adsorption. Using a simple model of an inkbottle pore we demonstrated that commonly assumed pore blocking effects do not play a role in hysteresis formation. We applied molecular dynamics simulation technique to a model of silica xerogel to reveal a good agreement between grand canonical Monte Carlo method and molecular dynamics approach. The nature of this agreement lies in the fundamental physics incorporated into the Metropolis algorithm. Finally, using lattice models of disordered porous media and a corresponding mean field theory, we explored various aspects of confined fluid behavior. In particular, we have shown that the developed lattice model and theory are capable of generating qualitatively correct adsorption isotherms, hysteresis loops and scanning curves. The theory also allowed us to investigate the nature of hysteresis. Hysteresis appears to be an outline of multiple metastable states confined within hysteresis loop. This creates a new angle to look at experimental data and provides a self consistent framework to study adsorption in various materials.